We have currently changed our resources from user accounts to resource room accounts in Exchange 2010 to prevent all day events from defaulting to "Free" under the free/busy tab. But now even if it shows up as busy or tentative, it still allows doublebooking even though "Allow conflicting appointments" has been disabled.

I don't know what else to do because that little message that says "Conflicts with another appointment" is not enough to discourage someone from double booking the appointment.